1. Field of the Invention
The present invention relates to a technique for measuring spectral responsivity of an image pick-up device such as a digital camera, a digital still camera or the like, and to a technique for configuring imaging data suitable for being utilized when an image picked up by the image pick-up device is outputted by an image output device.
2. Description of the Background Art
Conventionally, there has been a typical device and method of measuring color characteristic of an image pick-up device which has been described in Section 3, Clause 18 of xe2x80x9cInternational Standard IEC1146-1 Video cameras (PAL/SECAM/NTSC) Methods of Measurementxe2x80x94Part 1: Non-broadcasting single sensor camerasxe2x80x9d published by International Standard by IEC (International Electrotechnical Commission) in May, 1994 as an international standard.
FIG. 20 is a diagram showing application of the above-mentioned image pick-up device and method, illustrating a structure of a device for measuring a color reproducibility and gradation characteristic of a digital still camera used as an example of an image pick-up device. FIG. 20 shows an image pick-up device 1 and a test chart 20 which is a subject of the image pick-up device 1. FIG. 20 also shows an illumination light source 21 having a stable color temperature and for illuminating the test chart 20, and an image output device 15 for receiving data output from the image pick-up device 1, for example, a computer or the like.
FIG. 21 is a diagram showing a structure of the test chart 20 including, as reference colors, a gray scale 30 having white, black and grays changing stepwise from white to black, and several color charts 31 such as red, green, blue and the like. Examples of the color charts 31 have characteristics defined in Annex A, B of the above-mentioned International Standard.
It is assumed that an RGB value of each color chart 31 of the test chart 20 shown in FIG. 21 is well known and is a theoretical value (for example, if data has 8 bits, ideally, red has R=255 and G=B=0, green has G=255 and R=B=0, and blue has B=255 and R=G=0). By calculating differences (color differences) between the R, G and B values corresponding to each color chart 31 measured when the image of the test chart 20 is picked up by the image pick-up device 1 and the theoretical R, G and B values, the color reproducibility of the image pick-up device 1 can be evaluated. The gradation characteristic of the image pick-up device 1 can be found based on measured values acquired when the gray scale 30 changing stepwise from white to black is picked up by the image pick-up device 1.
However, an illuminance of the chart 20 illuminated by the illumination light source 21 is varied in positions on the chart 20. Therefore, even if the image of the same color chart 31 is picked up, a measured value is varied in positions on the chart 20. For this reason, there has been a problem in that uneven illumination should be corrected to obtain an accurate value.
Even under ideally even illumination, the measured values are varied due to a difference in a light quantity between a central portion and a circumferential portion, for example, depending on the characteristics of an image pick-up optical system of the image pick-up device even if the image of the same color chart 31 is picked up. Therefore, there has been a first problem in that the characteristics of the image pick-up optical system of the image pick-up device should be well known and should be corrected to obtain an accurate value.
Moreover, if the test chart 20 is a printed matter, a deterioration with age such as fading, discoloration or the like is caused. Therefore, there has been a problem in that it is hard to perform measurement with a high reproducibility.
Furthermore, there has been a drawback that the color difference between the measured R, G and B values for each color chart on the test chart 20 and the theoretical R, G and B values can be obtained but a spectral responsivity characteristic of the image pick-up device for each wavelength cannot be measured. The spectral responsivity characteristic of an image pick-up device refers to the response of the device to light as a function of light wavelength. In other words, the relationship between image signal intensity for the image pick-up device and light wavelength (i.e., spectrum dependency of an image signal) represents the spectral responsivity characteristic of the image pick-up device.
Another problem is that the image pick-up device cannot perform color correction with high precision for the images of general subjects with information obtained from only a limited variety of images.
There has been another drawback that a work for separately making a correspondence list or the like is required to correlate data measured by a color characteristic measuring device with an image picked up by the image pick-up device.
If the type of the illumination light source 21 (spectral distribution characteristic) is changed, the data corresponding to individual color charts picked up by the image pick-up device are also changed. Conventionally, there has not been means for accurately reflecting the individual characteristics of the illumination light source 21. Therefore, there has been a drawback that it is hard to perform color management of the image pick-up device including the illumination light source 21. For the above-mentioned reason, particularly, there has not been a method of accurately measuring the spectral distribution characteristic of the image pick-up device. Therefore, even if the spectral responsivity characteristic of the light source or the like is accurately measured, it is impossible to perform the color management effectively utilizing the spectral responsivity characteristic.
A first aspect of the present invention is directed to a method of measuring spectral responsivity characteristic of image pick-up device. The method comprises steps of: (a) preparing an image pick-up device having an image sensor including plural pixels arranged, a light source, and a spectroscope for spectrally separating a light from said light source and outputting separated light from an emitting terminal thereof; (b) picking-up an image of said emitting terminal which is outputting said separated light by said image pick-up device; (c) performing a correction to each of plural image signals corresponding to at least one color component and said plural pixels that are outputted by said image pick-up device during said step (b) so that a gradation characteristic of said image pick-up device is corrected from non-linearity to one closer to linearity; and (d) calculating, as a spectral responsivity characteristic for said separated light, a mean value of said plural image signals after said correction by averaging the same over a certain range within said plural pixels.
Preferably, varying a wavelength of said separated light, said step (b), (c), and (d) are repeatedly executed so as to obtain said spectral responsivity characteristic for plural wavelengths.
Preferably, in said step (c), a value of an inversion function of a function that expresses said non-linearity in said gradation characteristic is calculated for each of said plural image signals whereby said correction is performed.
Preferably, in said step (d), using a spectral characteristic that expresses an intensity of said separated light for each of said plural wavelengths, said mean value is corrected as said spectral responsivity characteristic so as to reduce an influence of difference in said intensity among said plural wavelengths.
Preferably, in said step (a), another light source for illuminating said emitting terminal is further prepared, in said step (b), illuminating said emitting terminal with said another light source, said image of said emitting terminal is picked-up by said image pick-up device, and in said step (c), in addition to said correction, another correction to subtract a signal component resulting from said another light source is performed to each of said plural image signals.
Preferably, in said step (a), a box that has a hole selectively opened where said separated light passes from said emitting terminal to said image pick-up device and an illumination light source for emitting such an illumination light as to reach said hole and a portion of a surface of said box surrounding said hole but not to reach said emitting terminal are further prepared, and said method further comprises a step of (e) sequentially fitting plural color chips having different reflectances in said hole, while picking-up images of said plural color chips with said image pick-up device, and thereby measuring said non-linearity in said gradation characteristic on a basis of image signals sequentially output by said image pick-up device.
A second aspect of the present invention is directed to a method of configuring imaging data. The method comprises steps of: (A) preparing a image-pick up device; (B) obtaining an image data that represents an image of a subject as an output signal of said image pick-up device; and (C) adding a data that represents a spectral responsivity characteristic of said image pick-up device to said image data.
Preferably, said spectral responsivity characteristic is obtained by applying the method according to a first aspect of the present invention to said image-pick up device prepared in said step (A).
Preferably, the method further comprises a step of (D) adding, to said image data, a conversion coefficient between a first signal obtained from said image-pick up device when an image of a color chart is picked-up by said image pick-up device under an illumination of a standard white color and a second signal obtained from said image-pick up device when an image of said color chart is picked-up by said image pick-up device under an illumination having a particular spectral distribution characteristic.
A third aspect of the present invention is directed to a method of configuring imaging data. The method comprises steps of: (A) preparing a image-pick up device; (B) obtaining an image data that represents an image of a subject as an output signal of said image pick-up device; and (C) adding, to said image data, a conversion coefficient between a first signal obtained from said image-pick up device when an image of a color chart is picked-up by said image pick-up device under an illumination of a standard white color and a second signal obtained from said image-pick up device when an image of said color chart is picked-up by said image pick-up device under an illumination having a particular spectral distribution characteristic.
According to the first aspect of the present invention, the spectral responsivity characteristic is obtained precisely.
According to the second aspect of the present invention, an image output device for receiving the imaging data can obtain an information needed for color reproducibility to adequately perform color correction such as color matching.
According to the third aspect of the present invention, an image output device for receiving the imaging data can reproduce an image of the subject with precise color reproducibility.
It is an object of the present invention to provide a technique to accurately measure a spectral responsivity characteristic without managing an uneven illuminance, a deterioration with age, color charts and the like on the test chart.
It is another object of the present invention to provide a technique to measure a spectral responsivity characteristic without accurately finding a spectral characteristic of illumination and the like.
It is yet another object of the present invention to provide a technique to obtain a spectral responsivity characteristic required to accurately perform color reproduction of a subject.
It is a further object of the present invention to provide a technique to always know color characteristics of an image pick-up device by adding the spectral responsivity characteristic obtained from the image pick-up device to image data and to perform color correction with high precision for a general image.
It is a further object of the present invention to provide a technique to obtain an accurate spectral responsivity characteristic which does not depend on a gradation characteristic of the image pick-up device.
It is a further object of the present invention to provide a technique to obtain image data capable of accurately displaying color reproduction of a subject in an image output device.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.